Hydraulic device



R. L. COOPER ETAL HYDRAULIC DEVICE Filed Feb. 7, 1965 BY @WWA/W ATTORNEYS Feb. 15, 1966 `motive fluid for the starting operation.

`surized by a. selector valve. `the spring to act on both sides of the zero displacement .position of the displacement control element while main- `taining its length within tolerable limits.

United States Patent Otihce .3,234,889 Patented Feb. 15, 1966 3,234,389 HYDRAULlC DEVICE Richard L. Cooper and John W. Fluno, Watertown, NX.,

assignors to The New York Air Brake Company, a corporation of New `lersey Filed Feb. 7, 1963, Ser. No. 256,924 3 Claims. (Cl. 1113-162) This invention relates to hydraulic motor-pump units intended for uses as engine starters. The co-pending application of James R. Parr, Serial N o.

216,191, led Aug. 10, 1962, discloses a hydraulic starting system including a motor-pump unit of the overcenter type and utilizing an accumulator as the source of The displacement control element of the motor-pump unit is urged. toward the tir-st of its maximum displacement positions by a biasing spring and is shifted toward the second maximum displacement position Iby a control motor which is vented and pressurized by a control valve that respends to the pressure at the high pressure port of the unit. One end of the biasing spring is seated on a movable spring seat which is shifted in opposite directions by a shifting motor that is selectively vented and pres- This arrangement allows The spring seat also carries a limit stop which :is rendered effective t-oprevent overcenter movement of the displacement control element when the shifting motor is pressurized and the spring is positioned for pumping operation.

In application Serial No. 216,191, the control valve is biased toward its vent position lby a valve spring and is shifted toward its pressurizing position by a pressure motor that responds to the pressure at the high pressure port yof the unit. The preload in the valve spring determines the magnitude of the predetermined pressure at which the control valve shifts from its venting to its pressurizing position and this predetermined pressure can be selectively raised and lowered by a pair of actuating motors, one of which shifts the seat of the valve spring to increase the spring load, and the other of which acts in opposition to and overpowers the valve spring. In the system illustrated in application Ser. No. 216,191, the motor-pump unit .is used as a starter for an auxiliary engine, and, after that engine is r-unning, as the source of lmotive iiuid for operating .a starter connected with a second or main eng-ine. The first actuating motor is vented and pressurized automatically when the start valve controlling flow from the motor-pump unit to the second starter is closed and opened, respectively. The second actuating motor, on the other hand, is vented and pres surized automatically when the auxiliary start valve controlling flow from the accumulator to the motor-pump unit is closed and opened, respectively. The system inc'ludes interconnected actuation circuits for the selector .valve and the auxiliary start valve so that the shifting motor and the control valve are simultaneously set for motoring .and pumping operation.

It is an object of this invention to .provide an improved vstarter system of the type mentioned above in which the selector valve is connected directly with and operated by the second actuating motor. This improvement insures `simultaneous adjustment of the `Settings of the shifting equipped with a iluid pressure biasing motor that is pressurized and vented simultaneously with the shifting motor and which aids the spring in urging the displacement control element toward `the first maximum displacement position. During the motoring operation, the shifting motor is vented and, therefore, so too is the biasing motor. Thus, during this phase of operation, the biasing motor has no effect. During the pumping operation, when the shifting motor is continuously pressurized, the biasing motor exerts a substantially constant biasing force on the displacement control element that augments the variable bias exerted by the spring. Therefore, as explained in Budzich Patent 3,063,381, granted Nov. 13, 1962, the response rate of the displacement control circuit is increased without altecting the control dilerential of the pressure compensator.

The preferred embodiment of the invention is described herein with reference to the accompanying drawing whose single ligure is a schematic diagram of the improved starter system, the cam plate of the rnotonpump unit being shown in the neutral or zero displacement position.

As shown in the drawing, the starting system includes an overcenter motor-pump unit 1 of the rotary cylinder barrel, longitudinally reciprocating piston type whose drive shaft 2 is connected in driving and driven relation with auxiliary engine 3 and which is arranged to receive motive fluid from a gas-charged accumulator 4 and to deliver motive fluid to a starter m-otor 5 whose drive shaft 6 is connected in driving relation with main engine 7. Motonpump unit 1 includes high and low pressure ports 8 and 9 and a cam plate 11 which is mounted for pivotal movement about the axis of trunnion 12. The low pressure port 9 of unit 1 is in continuous communication with reservoir 13 and the high pressure port 8 is connected with accumulator 4 along two parallel paths, one path including conduits 14, 14a and 14b and controlled by auxiliary .start valve 15, and .the other path including conduits 14, 14a, 14e and 14b and containing a check valve 16. High pressure port 8 also is connected with a hydraulic system including various power operated devices (not shown) through conduits 14 and 14d, and with the inlet port 17 of starter motor 5 through conduits 14, 14a, and 14e. The last mentioned connection is controlled by main Vstart valve 15. Discharge port 18 of starter motor 5 is in continuous communication with reservoir 13.

The auxiliary and main start valves 15 and 15 are o f the pilot-operated ty-pe disclosed in the application Serial No. 216,191 and each is spring biased to the closed position. When the pilot valve of either of the valves 15 and 15 is shifted from its venting to its pressurizing position, the connection between the piloted motor and reservoir 13 is interrupted and this motor is connected with the conduit upstream of the start valveA (that is, conduit 14h in the case of valve 15 and conduit 14a in the case of valve 15'). As .a result, the piloted motor shifts the valve 15 or 15', as the case may be, to the open position. Venting of either piloted motor allows the spring to close the start valve.

The displacement of motor-pump unit 1 is controlled by cam plate 11 which is biased in the counterclockwise direction about the axis of trunnion 12 by a combined spring and motor assembly 19. This assembly 19 cornprises a pair of telescoping cylinders 21 and 22 formed with spherical heads at their outer ends which are centered on the longitudinal axis of the cylinder and which are received in sockets formed in cam plate 11 and in the piston 23 of shifting motor 24. Mounted within cylinder 22 is a reciprocable piston 25 which abuts the end wall 26 of cylinder 21 and which, together with cylinder 22 and working chamber 27, defines a biasing motor 28. Although piston can be omitted and cylinder 21 used directly to dene the biasing motor, the illustrated arrangement is preferred because it permits the force exerted by the motor to be changed simply by varying the diameter of piston 25 and its mating bore in cylinder 22. Encircling the two cylinders 21 and 22 is a coil compression spring 29 whose opposite ends are seated on the ange 31 formed on cylinder 21 and on the seat 32 carried by cylinder 22, respectively. Seat 32 is provided with a longitudinal extension 33 which is arranged to abut the end of cylinder 2.1 when the two cylinders are moved toward each other. This extension 33 constitutes a limit stop for the cam plate 11 and, as will' appear below,it is designed so that during the pumping mode of operation it prevents the cam plate 11 from moving over center, i.e., beyond the zero displacement position.

Theshifting motor 24, includes the piston 23, a stationary cylinder 34, and a workingchamber 35. Piston 23 is designed to move between an illustrated irst position in which it abuts stop 36 and a second position in which its head 37 abuts shoulder 38 in order to position assembly 19 for the motoring and pumping operations, respectively. The working chambers 27 and 35 of motors 28 and 24, respectively, are interconnected by an axial passage 39 extending through piston 23 and the spherical head of cylinder 2-2.

Cam plate'11 of motor-pump unit 1 is moved in the lclockwise direction about the axis of trunnion 12 against the opposing bias of assembly 19 by a control motor 41 Vincluding piston 42 and working chamber 43. Inserted intopiston 42 is a rod 44 having an enlarged head 45 which is arranged to abut and dene with end wall 46 a stop that establishes the maximum displacement position of cam plate 11 on the pumping side of the zero displacement position. This maximum displacement position can be changed easily by varying the longitudinal width of head 45. Fluid is supplied to and exhausted from Working chamber 43 under the control of a pressure responsive control valve 47 with which it is connected by a conduit 48. This control valve 47 includes an inlet chamber 49 which is connected with high pressure port 8 of motor-pump unit 1 through conduits 14f and 14, an outlet chamber 51 which is connected with conduit 48an exhaust chamber 52 which is connected with reservoir 13, and a sliding valve plunger 53 formed with two `annular grooves 54 and 55 that dene three valve lands 56, 57 and 58. A at 59 extends across the entire width of land 56 and a stopped lat 61 extends part way across land 58. Valve plunger 53 is biased to the left toward the illustrated vent position, in which land 57 isolates outlet chamber 51 from inlet lchamber 49 and chambers 51 and 52 are interconnected across hat 61, by a coil compression spring 62. The valve plunger 53 is shifted to the right, rst to a lap position in whichlands 57 and 58 isolate outlet chamber 51 from chambers 49 and 52, and then to a supply position in which outlet chamber 51 is isolated from exhaust chamber 52 by land 58 and is interconnected with inlet chamber 49 through groove 54 and across flat 59, by the fluid pressure in inlet chamber 49 which acts upon `theleft end of the valve plunger.

The pressure in inlet chamber 49 required to shift valve plunger 53 to the lap position against the bias of spring 62 is termed the reference pressure, and the control valve 47 includes two devices for raising and lowering this reference pressure. The iirst device, which is used to raise the reference pressure above the normal value, comprises a movable spring seat 63 for spring 62 which is shiftable between the illustrated low spring-load position and a high spring-load position in which it abuts stop 64, and a piston motor including the right end of seat 63 and a working chamber 65. A conduit 66 connects working chamber 65 with the pilot valve portion of the main engine start valve 15' so that this working chamber is vented and pressurized whenever the main engine start valve is closed and opened, respectively. Control valve 47 is adjusted to establish its lowest reference pressure by a piston motor 67 comprising piston 6%, which abuts the left end of valve plunger 53, and a working chamber 69 which is connected with the pilot valve portion of auxiliary engine start valve 15 by conduit 71. Therefore, working chamber 69 is Vented and pressurized when valve 15 is closed and opened, respectively.

The piston 68 of motor 67 also serv-es as a valve plunger of a selector valve 72 that vents and pressurizes the Working chambers 27 and 35 of motors 25 and 24, respectively. For this purpose, piston 68 is formed with three grooves 73, 74 and '75 and three lands 76, 77 and 7S. The lands 76 and 78 are provided with tlats 79 and 81, respectively, that extend across their entire widths. The selector valve 72 also includes the inlet chamber 49 of the control valve 47, an exhaust chamber 32 which is connected with reservoir 13, and an outlet chamber 83 which is connected with working chamber 35 of motor 24 by a conduit 84. When auxiliary engine start valve 15 is opened to commence the starting cycle of auxiliary engine 3, piston motor 67 is pressurized to adjust control valve 47 to its lowest reference pressure and simultaneously to vent motors 24 and 28 along a path comprising conduit 84, outlet chamber 83, hat 79 and exhaust chamber 82. When auxiliary engine start valve 15 is closed, as it is during the pumping mode of operation of motor-pump unit 1, piston motor 67 is vented and piston 68 assumes the illustrated position in which land 77 blocks communication between chambers S2 and 83, and chamber 83 is connected with the inlet chamber 49 of control valve 47 through groove 75 and across hat 81. Thus, during the pumping operation, motors 24 and 28 are pressurized.

It will be noticed that the diameter of the head 68' of piston 68 is greater than the diameters of lands 76, 77 and 78. Because of this, piston 68 will always shift to the right when the pressure in working chamber 69 and in inlet chamber 49 are equal.

Like motor-pump unit 1, starter motor 5 is also of the rotary cylinder barrel, longitudinally reciprocating piston type. Preferably motor 5 is a variable displacement unit, but for simplicity, its controls are not illustrated. The co-pending application of Melvin L. Kent and Edward V. Manning, Serial No. 119,170, now Patent 3,170,450, tiled June 23, 1961, discloses a suitable design for this motor.

OPERATION At the commencement of the starting cycle, start valves 15 and 15 are closed so th-at motor-pump unit 1 is isolated from the accumulator 4, and starter motor 5 is isolated from motor-pump unit 1. Initially, ccumulator 4 is charged by a hand pump circuit (not shown). For purposes of the following discussion, it is assumed that the precharge pressure of accumulator 4 is 1500 psi., that the fully charged pressure of the accumulator is 3000 p.s.i., and that the three reference pressures of control valve 47 are 1500 p.s.i., 3G00 p.s.i., and 4000 p.s.i.

in order to start auxiliary engine 3, the operator opens auxiliary start valve 15 so that uid from accumulator 4 may ow to the high pressure port of motor-pump unit 1 through conduits 14h, 14a and 14. Simultaneously, uid at accumulator pressure is delivered to conduit 71 by the pilot portion of start valve 15 and thence to the working chamber 69 of motor 67. The pressure developed in this chamber shifts piston 63 to the right thereby automatically setting control valve 47 to its lowest reference pressure and venting working chambers 27 and 35 of motors 28 and 24, respectively, to reservoir 13` through conduit S4 and chamber 83, across at 79, and' through chamber 82. The pressure transmitted to inlet Chamber 49 of control valve 47, in conrrloinationwith, the

emesso shifting action of piston motor 67, causes valve plunger 53 to move to its supply position and allow fluid from the accumulator to pass to working chamber 43 of control motor 41 along a path compris-ing conduits 14 and 141, inlet chamber 49, flat 59, groove 54, outlet chamber 51, and conduit 4S. As a result, control motor 41 moves cam plate 11 in the clockwise direction to its maximum stroke position on the motoring side of neutral and maintains it in that posit-ion against the opposing bias of spring 29.

The fluid delivered to high pressure port 8 causes motorpump unit 1 to develop torque and accelerate engine 3. As engine 3 accelerates, the pressure of the iuid delivered by accumulator 4 progressively decreases from 3000 p.s.i. to 1500 p.s.i. Since the combined forces developed by motor 67 -and by the iiuid pressure in chamber 49 acting upon the left end of valve plunger 53l are suiiicient to hold the valve plunger S3 in the supply position at pressures above 1500 p.s.i., working chamber 43 remains pressurized and control motor 41 holds cam plate 11 in the maximum displacement position for the entire starting cycle. The parts of the system are so proportioned that engine 3 will rea-ch ignition speed before accumulator pressure begins to approach 1500 p.s.i. and will reach or even exceed starter cut-out speed (i.e., the speed at which the engine is capable of developing sufficient power to accelerate itself) by the time accumulator pressure reaches 1500 p.s.i. Therefore, when the accumulator pressure decreases to 1500 p.s.i., and the accumulator is fully discharged, engine 3 will be running and accelerating at a rate greater than that of motor-pump uni-t 1. Consequently, at this time system pressure will decrease momentarily below 1500 p.s.i. and spring 62 will shift valve plunger 53 to the vent position and cause it to exhaust working chamber 43 to reservoir 13 along a path cornprising conduit 48, outlet chamber 51, i'l'at 61 and chamber 52. As a result, spring 29 moves cam plate 11 in the counter clockwise direction a slight distance beyond the zero displacement position so that motor-pump unit 1 commences to' discharge tluid under pressure from high pressure port` 8.: This pumping act-ion of motor-pump unit 1 restores system pressure to 1500 p.s.i. and, when the displacement of this unit equals the rate of leakage from the system, valve plunger 53 will move to the right to the lap position and hydraulically lock control motor 41. Motorapump unit 1 is now serving as a pressure compensated pump that establishes a maximum system pres- 4sure of 1500 p.s.i. Since this pressure is relatively low and the displacement of motor-pump unit 1 is quite small, the load on engine 3 is small and the engine may accelerate rapidly to the speed at which it develops sufficient power to start main engine 7.

At this point in the cycle the operator closes auxiliary start valve and thereby causes the pilot portion of this valve to vent working chamber 69. Spring 62 now shifts control valve plunger 53 to its vent position against the opposing force developed by the uid pressure in inlet chamber 49, and this same tluid pressure acting on the right end of piston 68 returns that piston to its illustrated pressurizing position in which working chambers .27 and 35 are connected with high pressure port 8 through a path including conduit 84, chamber 83, flat 81, chamber 49, and conduits 14f and 14. The pressure in working chamber 35 moves piston 23 to the left against shoulder 33 thereby causing it to move assembly 19 to the left to its `pumping position. The force developed by motor 23 in combination with spring 29 moves cam plate 11 in the counterclockwise direction about the axis of trunnion 12 lto the maximum displacement position on the pumping 6 prevent movement of cam plate 11 in the clockwise direction beyond the neutral position.

The fluid discharged by unit 1 now passes through conduits 14, 14a, 14C and 14h to accumulator 4 and the accumulator is recharged to 3000 p.s.i. When the accumulator 4 is fully charged, and assuming there is no other demand for hydraulic fluid, system pressure tends to rise above 3000 p.s.i. and valve plunger 53 of the control valve 47 shifts to its supply position. Control motor 41, therefore, becomes effective to move cam plate 11 toward the zero displacement position against the opposing bias of spring 29 and biasing motor 2S. lnasmuch as these systems always have some leakage, the cam plate 11 will come to rest, and control motor 41 will be hydraulically locked, when system pressure is restored to 3000 p.s.i. and the displacement of motor-pump unit 1 equals the rate of leakage.

In order tol start main engine 7, the operator opens main start valve 15 so that fluid discharged by motorpump unit 1 can ow to starter `motor S through conduits 14, 14a and 14e. Opening of this ilow path imposes a demand on motor-pump unit 1 that produces a momentary drop in system pressure below 3000 p.s.i. Therefore, valve plunger 53 shifts to its vent position and spring 29 and biasing moto-r 2S move cam pla-te 11 in the counterclocltwise direction toward the maximum displacement position on the pumping side of neutral. Simultaneously with the opening of main start valve 15', the pilot portion of this Valve interconnects conduits 14a and -66 so that fluid at system pressure is transmitted to working chamber 65. The pressure in this chamber is effective to shift seat 63 to the lett into engagement with stop 64 with the result that the preload in spring 62 is in creased. This action automatically raises: the reference pressure of control valve 47 to 4000 p.s.i. tarter motor 5 now commences to accelerate engine 7 to its cut-out speed. i

After main engine 7 has been started and reaches cutout speed, main start valve 15 is closed and conduit 66 is again vented to reservoir 13. Spring 62 now moves seat 63 back to its low spring-load position thereby reducing the reference pressure of control valve 47 to 3000 p.s.i. Upon this decrease in reference pressure, the ilutid pressure in inlet chamber 49 becomes effective to immediately move valve plunger 53 to the supply position (assuming that the system is imposing no demand on motor-pump unit 1 at this time). Control motor 41, therefore, moves cam plate 111 in the clockwise direction to a reduced displacement position in which the displacement of motorpump unit 1 equals the rate of leakage and system pressure is maintained at the 3000 p.s.i. level.

Motor-pump unit 1 is now available to supply the various demands connected with conduit 14d and it will be understood that in handling these demands, as well as those of starter rnotor 5, the compensator including control valve 47, control motor 41 and assembly 19, will vary the angular position of cam plate 11, and consequently, the displacement of motor-pump unit 1, in inverse rel-ation to system pressure so that the pressure is maintained substantially constant at the selected reference pressure. It also will be understood that throughout the pumping operation of motorepu-mp unit 1, the biasing motor 28 is pressurized and thus maintains at a high level the shifting forces acting on cam plate 11. This insures rapid pump response. Stop 3-3, which also is continuously effective du-ring pumping operation, prevents the compensator from moving cam plate 11 back to the motoring side of neutral.

It might be mentioned here that in some cases, for example, when the ambient temperature is relatively high, the torque demand of engine 3 may be such that the entire capacity of accumulator 4 is not required during the starting cycle of this engine. In these cases, engine 3 will reach starter cut-out speed when accumulator pressure is still considerably above (for example, several hundred p.s.i. above) the lowest reference pressure of 1500 p.s.i.

Under these conditions, the operator may interrupt motoring operation of motor-pump unit 1 by closing auxiliary start valve 15. This action results in shifting of selector valve 72 to its pressurizing position and in adjustment of control valve t7 to its intermediate reference pressure of 3000 p.s.i. Therefore, valve plunger 53 of the control valve 47 moves to the vent position and assembly 19 pivots cam plate 11 in the counterclockwise direction to a small displacement position on the pumping side of neural. Motor-pump unit 1 now operates as a pump compensated to 3000 p.s.i. in the manner previously described. in the alternative, the operator may allow motor-pump unit 1 to accelerate engine 3 to a speed materially higher than starter cut-out speed and pe-rmit the controls to shift cam plate 11 in the counterclockwise direction automatically as in the normal case first described. However, in this event, the motor-pump unit 1 must be designed to withstand the overspeed condition.

ln addition to Parr application Serial No. 216,191 mentioned earlier, certain features of the preferred embodiment of the present invention are also disclosed and claimed in the co-pending applications of Tadeusz Budzich and Edward V. Manning, Ser. No. 180,927, now Patent 3,153,899, filed Mar. 12, 1962, Melvin L. Kent and Mike Prewarski, Ser. No. 111,721, now liatent 3,116,595, filed May 22, 1961, and Edward V. Manning and John P. Mentink, Ser. No. 92,052, now Patent 3,106,057 filed Feb. 27, 1961.

As stated previously, the drawing and description relate only to the preferred embodiment of the invention. Since many changes can be made in the structure of this embodiment without departing from the inventive concept, the following claims should provide the sole measure of the scope of the invention.

What we claim is:

1. In an overcenter motor-pump unit of the rotary cylinder barrel, longitudinally reciprocating piston type having `a housing containing high and low pressure ports, and in which the pistons are moved on their discharge strokes by a cam plate which is angularly shiftable between first and second maximum displacement positions on opposite sides of a zero displacement position, the improvement which comprises (a) a first piston motor including a working chamber and first and second relatively movable members, one member being a piston and the other being a cylinder, the first member being in operative engagement with the cam plate and the motor being arranged to shift the cam plate in the direction of the first maximum displacement position;

(b) a pair of opposed spring seats, one carried by each of said members of the rst piston motor;

(c) a coil compression spring reacting between the spring seats and acting through the first member of the first piston motor to urge the cam plate in the direction of the first maximum displacement position;

(d) a second piston motor including a working chamber and first and second members, one member being a piston and the other being a cylinder, the first member being stationary and fixed to the housing and the second being connected with the second member of the first piston motor and movable between first and second positions lin a direction that causes the first piston motor to shift the cam plate in the direction of the first maximum displacement position;

(e) stop means carried by the second member of the first piston motor and limiting relative movement of the members of this motor in a direction that compresses the spring, the stop means being arranged so that when the second piston motor is in its second position the members of the first motor prevent movement of the cam plate in the direction of the second maximum displacement posit-ion beyond the zero displacement position;

(f) a fluid pressure control motor acting on the cam plate in opposition to the first piston motor and the spring;

(g) means for varying the pressure in the control motor; and

(h) means for simultaneously varying the pressure in the working chambers of the rst and second piston motors,

(i) the second piston motor having an effective area sufiicient to develop a force that holds the second member of this motor in its second position against the opposing force developed by the control motor.

2. The improvement defined in claim 1 in which (a) the means for varying the pressure in the control motor' comprises (l) a control valve having an inlet passage connected with the high pressure port of the motorpump unit, an outlet passage connected with the control motor, and an exhaust passage and including a movable valve member shiftable between first and second positions in which, respectively, the outlet passage is connected with the exhaust passage and the inlet passage and having an intermediate lap position in which the outlet passage is isolated from both the exhaust passage and the inlet passage;

(2) a spring biasing the valve member toward the first position;

(3) a movable spring seat for the spring shiftable between low spring load and high spring load positions;

(4) means responsive to the pressure in the inlet passage for urging the valve member toward the second position;

(5) a first actuating motor acting in opposition to the spring;

(6) a second actuating motor connected with the movable spring seat and arranged to move the seat toward the high spring load position; and

(7) means for energizing and de-energizing the first and second actuating motors; and

(b) the means for simultaneously varying pressure in working chambers of first and second piston motors comprises (1) passage means interconnecting the working chambers of the first and second piston motors;

(2) a selector valve having an inlet passage connected with the high pressure port, an exhaust passage and an outlet passage connected with the working chamber of one of the first and second piston motors and including a movable valve member connected with the first actuating motor and having first and second positions in which, respectively, it connects the outlet passage with the inlet and exhaust passages, the valve member of the selector valve being moved rom its first to its second position as the first actuating motor moves the valve member of the control valve toward its second position.

3. In a hydraulic starting and pumping system of the type including an overcenter motor-pump unit having high and low pressure ports and a displacement control element shiftable between first and second maximum displacement positions on opposite sides of a zero displacement position, a compression spring urging the displacement control element toward the first maximum displacement position, a movable seat for the spring, stop means for selectively preventing and permitting movement of the displacement control element in the direction of the second maximum displacement position beyond the zero displacement position, a first motor which when pressurized shifts the spring seat in the spring-compressing direction and renders the stop means effective to prevent movement beyond the Zero displacementposition, a control motor for moving the displacement control element toward `the second maximum displacement position, a control valve responsive to the pressure at the high pressure port for venting and pressurizing the control motor as the pressure falls below and rises above a predetermined value, an actuating motor which when pressurized urges the control valve in the direction to` pressurize the control motor and thus lower said predetermined value, means for selectively pressurizing and venting the actuating motor, and selector valve means for selectively venting and pressurizing the first motor, the improvement which comprises means interconnecting the selector valve means and the actuating motor so that References Cited by the Examiner UNITED STATES PATENTS 3,116,595 1/1964 Kent et al. c 103--162 3,153,899 10/1964 Budzich et al. 10S-162 DONLEY J. STOCKING, Primary Examiner.

LAURENCE V. EFNER, Examiner. 

1. IN AN OVERCENTER MOTOR-PUMP UNIT OF THE ROTARY CYLINDER BARREL, LONGITUDINALLY RECIPROCATING PISTON TYPE HAVING A HOUSING CONTAINING HIGH AND LOW PRESSURE PORTS, AND IN WHICH THE PISTONS ARE MOVED ON THEIR DISCHARGE STROKES BY A CAM PLATE WHICH IS ANGULARLY SHIFTABLE BETWEEN FIRST AND SECOND MAXIMUM DISPLACEMENT POSITIONS ON OPPOSITE SIDES OF A ZERO DISPLACEMENT POSITION, THE IMPROVEMENT WHICH COMPRISES (A) A FIRST PISTON MOTOR INCLUDING A WORKING CHAMBER AND FIRST AND SECOND RELATIVELY MOVABLE MEMBERS, ONE MEMBER BEING A PISTON AND THE OTHER BEING A CYLINDER, THE FIRST MEMBER BEING IN OPERATIVE ENGAGEMENT WITH THE CAM PLATE AND THE MOTOR BEING ARRANGED TO SHIFT THE CAM PLATE IN THE DIRECTION OF THE FIRST MAXIMUM DISPLACEMENT POSITION; (B) A PAIR OF OPPOSED SPRING SEATS, ONE CARRIED BY EACH OF SAID MEMBERS OF THE FIRST PISTON MOTOR; (C) A COIL COMPRESSION SPRING REACTING BETWEEN THE SPRING SEATS AND ACTING THROUGH THE FIRST MEMBER OF THE FIRST PISTON MOTOR TO URGE THE CAM PLATE IN THE DIRECTION OF THE FIRST MAXIMUM DISPLACEMENT POSITION; (D) A SECOND PISTON MOTOR INCLUDING A WORKING CHAMBER AND FIRST AND SECOND MEMBERS, ONE MEMBER BEING A PISTON AND THE OTHER BEING A CYLINDER, THE FIRST MEMBER BEING STATIONARY AND FIXED TO THE HOUSING AND THE SECOND BEING CONNECTED WITH THE SECOND MEMBER OF THE FIRST PISTON MOTOR AND MOVABLE BETWEEN FIRST AND SECOND POSITIONS IN A DIRECTION THAT CAUSES THE FIRST PISTON MOTOR TO SHIFT THE CAM PLATE IN THE DIRECTION OF THE FIRST MAXIMUM DISPLACEMENT POSITION; 